WASHINGTON — President Barack Obama, who said his “one mandate” in a second term was to help middle-class families, takes the oath of office with many barriers to raising most Americans' living standards.

Most Americans started this year with a cut in take-home pay as Congress let a temporary 2-percentage-point reduction in payroll taxes expire. Workers' own leverage to gain wage increases will be limited for years by competition from the swollen ranks of jobless Americans as forecasters expect the unemployment rate to remain at or above 7 percent through 2014.

Besides a struggling global economy, Obama also faces united Republican opposition to his agenda and pressure to slash federal spending.

“The president is still fundamentally on the defensive,” said Damon Silvers, policy director for the AFL-CIO labor federation. “His fundamental task is to avoid being forced into austerity measures that will hurt the economic recovery or the capacity of our government to serve the people.”

Reversing decades-long slide

The pieces of Obama's economic agenda most likely to raise middle-class living standards in the near term are “the parts that look like they're not going anywhere,” said Larry Katz, a labor economics professor at Harvard University. These include plans for an infrastructure bank and tax breaks for new hires.

“Left by itself,” Katz said, the economic recovery is likely to continue generating “tepid, small improvements” in middle-income workers' living standards “but not enough to really offset the poor performance since the late 1990s.”

Middle-class workers – defined by one measure as households with annual incomes within 50 percent of the national median, or between about $25,700 and $77,000 – have faced a four-decade-long erosion of living standards. It was interrupted by a rare period of rising earning power during the fast-growing economy of President Bill Clinton's second term, Katz said.

Real median household income soared to $54,932 in 1999 from $50,661 in 1996 based on constant 2011 dollars, an all-time high since the U.S. Census began reporting the data in 1967.

By contrast, median household income in November was $51,310 – $3,850 lower than when Obama took office in January 2009, according to an analysis of census data by Sentier Research, an economic-consulting firm in Annapolis, Md.

In Clinton's second term, gross domestic product grew at an average 4.3 percent rate and the unemployment rate averaged 4.4 percent. That created a tight labor market that provided workers leverage to press for higher wages.

Clinton also won congressional passage of a two-step increase in the minimum wage, which took effect in 1996 and 1997, creating additional upward pressure on pay; the federal minimum wage hasn't gone up since 2009.

The gains of the late-1990s are eclipsed by decades of downward pressure on pay.

Median wages for men between 25 and 64 dropped 19 percent – to $34,000 a year – from 1971 to 2011, after accounting for inflation, according to an analysis by Michael Greenstone, a Massachusetts Institute of Technology economics professor.

The impact of the decades-long slide in wages initially was cushioned in many families by the increasing number of women who went to work, and later by the home equity that families borrowed against during the run-up in housing prices before the real estate bubble burst.

Competition from lower-wage workers overseas, technological advances that allow factories and offices to produce more with less labor and declining union membership all have combined to squeeze the pay of the moderately skilled workers who are the backbone of the middle class.

Myriad plans to try and help

Obama made the economic security of middle-class Americans the central theme of his re-election campaign. At a news conference after his re-election, he said his single mandate was “to help middle-class families and families that have been working hard to try to get into the middle class.”

Greenstone, a former chief staff economist for Obama's Council of Economic Advisers, said the president's health care law, which will take full effect in 2014, will “have a huge impact on economic security.” Its insurance subsidies for lower-income families and access to coverage with no exclusion for pre-existing conditions will reduce the risk of financial ruin from health problems, he said.

Amy Brundage, a White House spokeswoman, said Obama “will continue to fight to build on the economic progress we have made by investing in the things that spur growth” and protect the middle class.

Obama has voiced support for other goals – a simplified tax system with lower rates and fewer breaks for the wealthy, raising the number of college graduates, a major infrastructure program, an energy policy addressing climate change; expanded promotion of exports, and incentives favoring manufacturing.

“I know where their hearts are: They want to do all these things,” said William Galston, a former domestic policy adviser to Clinton. Yet, he added, “There's the real agenda and the notional, aspirational agenda.”